This is a misunderstanding of urpmi work and mono package organization. We don't have mono-4.0 package in our repositories (and you won't find it in rpmdrake. We have only mono-3.12.1 which provides "mono-4.0", but this provides stands for CLR version (that is, mono-4.0 means that this package supports CLR 4), not for the mono version.
I know these provides are confusing, but we have to keep them for backward compatibility. And finally, this will confuse only users who run urpmi directly, but I guess once you use a command line package manager you should be ready for its tricks:)